1. Field of the Invention
This invention relates generally to a hemming press and, more particularly, to a hemming press employing electric servomotors and hydraulic cylinders for actuating a press ram, where the servomotors provide a prehem pressure and the hydraulic cylinders provide a final hem pressure.
2. Discussion of the Related Art
It is common practice, particularly in the automotive industry, to join two preformed metal panels together into a unitary structural unit by a hemming process using a hemming press. A vehicle door is one unit that is typically manufactured in this manner. Particularly, the vehicle door typically includes an outer metal panel and an inner metal panel that are hemmed together to provide the complete door unit. Before the hemming process is performed, the outer metal panel is preformed to include an outer edge flange formed perpendicular to the main portion of the outer panel. This preforming is customarily done in a stamping operation. The preformed inner and outer panels are aligned relative to each other in a press die. The hemming process moves a press ram to fold the preformed flange around an edge portion of the inner metal panel. It is important that the hemming process provides a firm, vice-like grip of the outer panel flange with the edge portion of the inner panel while maintaining the door shape and dimensions within prescribed tolerances.
Known hemming procedures are generally two stage processes. In the first stage, the outer panel with the preformed flange is placed in the press die and the inner metal panel is placed on top of the outer panel so that the edge portion aligns with the flange. The hemming press applies an exterior force to the preformed flange of the outer metal panel at an angle generally perpendicular to the flange causing the flange to bend downward toward the inner panel. This step is generally referred to as the prehemming stage. The second stage provides a second force generally parallel to the original orientation of the flange to cause the partially bent flange to bend an additional amount to contact the inner panel and complete the folding process. This step is generally referred to as the final hemming stage.
This two stage hemming process is generally done using separate sets of dies or tools. The required tooling for such an operation is rather massive, costly and space consuming. Additionally, a two stage hemming process usually requires a transfer operation to transfer the panels to be hemmed from the first stage tooling to the second stage tooling.
The prior art generally relates to improving the hemming process from a two-stage process to a one-stage process. Examples of advances in this area may be found in U.S. Pat. No. 5,083,355 entitled xe2x80x9cHemming Apparatusxe2x80x9d issued Jan. 28, 1992, to Ernest A. Dacey, Jr.; U.S. Pat. No. 5,150,508 entitled xe2x80x9cHemming Machine and Methodxe2x80x9d issued Sep. 29, 1992, to Lucen St. Denis; U.S. Pat. No. 5,272,903 entitled xe2x80x9cHemming Machinexe2x80x9d issued Dec. 28, 1993 to Owen C. Evans; U.S. Pat. No. 5,454,261 entitled xe2x80x9cHemming Machine and Method of Operationxe2x80x9d issued Oct. 3, 1995 to Jon R. Campian; U.S. Pat. No. 5,457,981 entitled xe2x80x9cHemming Pressxe2x80x9d issued Oct. 17, 1995 to Gerald A. Brown et al.; U.S. Pat. No. 5,647,243 entitled xe2x80x9cDevice for Hemming Elements of Pressed Sheet Metalxe2x80x9d issued Jul. 15, 1997 to Antonio Zampini; U.S. Pat. No. 5,740,691 entitled xe2x80x9cHemming Machinexe2x80x9d issued Apr. 21, 1998 to Josef Kovarovic et al.; and U.S. Pat. No. 5,746,083 entitled xe2x80x9cHemming Machinexe2x80x9d issued May 5, 1998 to Josef Kovarovic et al. This art typically focuses on the use of cams external to the hemming press, pre-hem and final hem die blocks and methods for forming a unitary structure within a single hemming press. Further improvements can be made to improve the hemming process.
In accordance with the teachings of the present invention, a hemming press is disclosed that includes a crown and a base plate separated by four corner posts. A press ram is slidably mounted on the posts between the crown and base plate. A first die is mounted to a lower surface of the press ram and a second die is mounted to an upper surface of the base plate. Workpieces to be hemmed are placed in the second die, and the press ram is moved from an upper home position to a lower hem position to perform the hemming process. Pre-hemming and final hemming steps are performed by one continuous stroke of the press ram.
At least one electric servomotor is mounted to a top surface of the crown, and is mechanically coupled to a linear actuator, such as a roll screw, extending through the crown. The linear actuator is rigidly mounted to a top surface of the press ram. One or more pneumatically powered hydraulic actuators are also mounted to the crown, where each actuator includes a rod that extends through openings in the press ram. The linear actuators move the press ram from the home position through a prehem position to the start position of the final hem. At the position for the start of the final hem, the rods are positioned above a top surface of the press ram. Suitable actuation devices position blocks over the openings in the press ram to provide a surface that the rods can push against. Activation of the hydraulic actuators push the press ram through the final hem position under suitable pressure to perform the hem. The contact point between the rods and the blocks is a floating contact to accommodate transverse forces on the press ram during the hemming process. The servomotors are reversed to return the press ram to the home position.